<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body>div{
            width: 200px;
            height: 200px;
            background-color: greenyellow;
            overflow: hidden;/*解决粘连问题*/
            border: 2px solid red;
            border-radius:100px ;
        }
        div>div{
            width: 50px;
            height: 50px;
            background-color: red;
            margin-left: 50px;
            /*当元素的上边距和上级元素的上边缘重叠时,给元素添加上外边距会粘连*/
            margin-top: 50px;
        }
        body{
            margin: 0;/*去掉自带的8个像素外边距*/
        }
        *{
            margin: 0;/*去掉全部的外边距*/
        }
    </style>
</head>
<body>
<div>
    <div></div>
</div>
<h1>我是h1</h1>
<p>这是p标签</p>
<ul>
    <li>aaa</li>
    <li>aaa</li>
    <li>aaa</li>
</ul>
</body>
</html>